Electro-Optics - Applied Optics Fundamentals and Device Applications - page 55

Electro-optic modulator response times are much more consistent with LSI
data requirements. The electro-optic effect in certain materials is in the pico-
second range and therefore does not limit the modulation bandwidth. The
only limiting actors are the propagation times in the electro-optic crystals
and the drive circuitry involved. For a modulator or reasonable drive volt-
age requirements, a bandwidth of approximately 2 GHz can be achieved at a
wavelength of 0.633 μm in LiNbO 3 [83]. This type of electro-optic modulator
also compares favorably in its dimensionality. Thus, electronically addressed
electro-optic modulators are very useful for modulation of continuous
sources within LSI interconnect systems.
2.9 AssessmentofInterconnectSystemArchitectures:
OpticalNetworkingArchitectures
Network concepts for interconnecting a large number of computing ele-
ments are numerous, including rings, banyan, Clos, Omega, Benes', trees,
spanning bus hypercubes, de Broijn, crossbar, gather scatter, cylinder, and
other permutation networks. These networks must be compared to arrive
at the most efficient network for interconnecting, given the ranges of data
devices for a target application. “Efficient” in this context means not only
the minimization of circuit complexity, delay, conflict at a node, synchroni-
zation, and coding, but also the ease of implementation in terms of optical
interconnect technology.
There are several input issues that must be considered with high-speed
optical interconnects. Many of these networks operate using burst-mode
transmission. At very high-speeds, low levels of synchronization error or
jitter are much more difficult to achieve if burst-mode transmission is used
rather than synchronous operation. Jitter may be reduced by buffering, but
at the cost of added network delay; therefore, synchronous operation is pre-
ferred to burst-mode transmission. Appropriate encoding schemes must be
used for optical interconnects. Binary, balanced, and nonalphabetic codes
are preferred.
2.9.1 Direct Relay Interconnects
The most direct method of interconnecting ICs from board to board is to inter-
connect the various data elements using LED arrays butt coupled to arrays or
buses of fiber. This architecture can become inadequate, however, with large
fan outs due to power requirements. Another direct-relay interconnect is the
fiber star coupler. In this approach, fanout and selective interconnection is
achieved from N ports to N other ports. This is done by twisting jacketed